In precision systems that perform operations on workpieces and the like, the workpiece is placed on, held by, and moved as required by a stage or other device that undergoes controlled motion relative to a tool, optical system, energy source, or other implement that performs the operation(s) on the workpiece. Such motion can be achieved by any of various actuators. Linear motors and planar motors have become favored for this purpose due to their wide range of motion, accuracy, precision, reliability, and simplicity.
Ongoing development of microlithography as applied to the fabrication of microelectronic devices has resulted in progressively stricter demands on performance and accuracy of microlithography systems. In addition, economic viability of fabrication procedures requires that microlithography systems perform with higher throughput. The need for higher throughput has imposed corresponding needs for greater velocity and acceleration of moving components of the systems, including the stages.
Satisfying demands for greater velocity and acceleration usually results in the stages (e.g., wafer stage) running at higher duty-cycle ratios, which typically results in increased power consumption by the actuators (e.g., linear motors and planar motors) producing stage motion. Greater power consumption often results in the actuators running hotter and contributing more heat to nearby components such as amplifiers in the circuitry used for actuator control. Time-variable changes in temperature of a motor and/or its control circuitry also causes corresponding variations in the motor-force constant and hence in the accuracy of stage performance, particularly over time.